Transcription

1 IEEE WLANs (WiFi) Part II/III System Overview and MAC Layer

2 Design goals for wireless LANs (WLANs) Global, seamless operation Low power for battery use No special permissions or licenses needed to use the spectrum for building WLAN (vs. Billions of dollars spent for 3G networks spectrum worldwide) Robust transmission technology Easy to use for everyone, simple management e.g. Simplified spontaneous cooperation at meetings Protection of investment in wired networks, e.g. compatible with Ethernet Security no one should be able to read my data, no one should be able to collect user profiles Safety (low radiation) Transparency concerning applications and higher layer protocols

5 ESS Architecture of an Infrastructure network and terminology BSS LAN STA 1 Access Point Portal Distribution System Access Point 802.x LAN STA 2 BSS 2 STA LAN Station Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point Access Access Point (or Base Station) station integrated into the wireless LAN and the distribution system Basic Basic Service Set (BSS) group of stations using the same radio frequency (channel) Distribution System interconnection network for multiple BSSes to form one logical network, the so-called Extended Service Set (ESS) PortalPortal Bridge/Gateway to other (wired) networks

6 STA Architecture of an ad-hoc (infrastructureless) network LAN IBSS 1 STA 3 STA 2 IBSS 2 STA 5 STA LAN Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS( IBSS): group of stations using the same radio frequency No Access Points, every STA is equal Direct (single-hop) communication within a limited range ; i.e. no multi-hop routing

8 802.11: Channels, association b: 2.4GHz-2.485GHz 2.485GHz spectrum divided into 11 channels at different frequencies ; Actually quite few of the 11 channels are totally non-overlapped overlapped Network administrator chooses frequency for each AP Interference possible: channel can be same as that chosen by neighboring AP! Better to have frequency assignment planning Station STA (or host): must first authenticate and then associate with an AP before it can start transmitting data How a STA get started? The STA scans channels, listening for beacon frames containing the ESS s name (SSID) and the MAC address of the AP (BSSID) selects AP to associate with may perform authentication before association After association is done, STA will typically run DHCP to get IP address in AP s s IP subnet (if STA does not have an IP addr already, or if it has moved to a new IP subnet)

10 Other IEEE Wireless LAN Standards a 5-66 GHz range up to 54 Mbps Disadvantages: stronger shading due to higher frequency Using different frequency ranges than b => extra hardware to support dual mode of a or b => Bad for backward compatiblility g GHz range Backward compatible with b up to 54 Mbps a and g use OFDM in physical layer All use CSMA/CA for multiple access All support AP (base-station), station), (infrastructure) and ad-hoc network modes

15 IEEE Multiple Access Control (MAC) Avoid collisions: 2 + nodes transmitting at same time : CSMA - sense before transmitting don t t collide with ongoing transmission by other node : no collision detection! difficult to receive (sense collisions) and transmitting at the same time because (TDD operation) transmission and reception are at the same frequency => the station s s receiver s s will be overwhelming by it s s own transmitting signals and thus can t t hear the relatively weak signals sent by the others In general can t t sense all collisions anyway, e.g. The Hidden terminal problem => Design Goal: avoid collisions: CSMA/C(ollision)A(voidance C(ollision)A(voidance) A C B A B C A s signal strength space C s signal strength

16 MAC CSMA/CA access method DIFS DIFS contention window (randomized back-off mechanism) medium busy direct access if medium is free DIFS slot time next frame t station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) time) if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

20 To Further Avoiding collisions Idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they re short) BS broadcasts clear-to to-send CTS in response to RTS RTS heard by all nodes sender transmits data frame other stations defer transmissions Avoid data frame collisions completely using small reservation packets!

27 DFWMAC-PCF (cont d) t 2 t 3 t 4 PIFS SIFS CFend point coordinator D 3 D 4 SIFS wireless stations U 4 stations NAV NAV contention free period contention period t Note: Since SIFS < PIFS, if User 3 has something to send, it would have done so before the Point coordinator s waiting period of PIFS is over ; not hearing from user 3 by then means the Point coordinator can safely proceed to User 4

28 Synchronization using a Beacon (infrastructure) beacon interval Access Point medium B B B B busy busy busy busy value of the timestamp B beacon frame t The AP broadcasts a Beacon message periodically which contains the reference time-stamp value ; Each station adjusts/tunes its local clock using the value of the time-stamp in the Beacon as the reference.

29 Synchronization using a Beacon in an Ad-hoc Network (i.e. No AP) beacon interval station 1 B 1 B 1 station 2 B 2 B 2 medium busy busy busy busy value of the timestamp B beacon frame random delay For each station: (1) Set a random delay timer ; (2) If a Beacon from another station is received before timer expires Adjust local clock using time-stamp in the received Beacon as reference else (i.e. no Beacon from others heard before timer expires) Upon timer expires, broadcast a Beacon using local clock to generate the time-stamp ; (to be used by all other stations as time reference) t

30 Power management Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) (see previous 2 pages) stations wake up at the same time Infrastructure Traffic Indication Map (TIM) list of unicast receivers transmitted by AP When unicast receiver wakes up and hear it has incoming frames buffered by the AP, it sends PS-poll to AP to ask for delivery Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP Unicast Polling mechanism does not work for Multicast/Broadcast from AP ; instead, it s s each STA s responsibility to wait up every DTIM-interval to check for Multicast/Broadcast from AP Ad-hoc Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability?)

38 Review on Naming and Addressing and Translation Mechanisms Given the Domain-name name of the destination host, e.g. the end-host ask an Domain Name System (DNS) server to translate it to an IP address, e.g This IP address is put into the destination IP address field of the packets sent to the destination host ; this destination IP address is used for routing table lookup along the path When the packet arrives at the destination network (the destination IP subnet), e.g. the LAN segment connected to the destination host, the last-hop router use the Address Request Protocol (ARP) to translate the destination IP address to the MAC address of the ethernet network interface card (NIC) on the destination host This MAC address is used as the destination address to deliver the t Ethernet frame to the destination host via L2 forwarding

41 802.11: mobility within same subnet H1 remains in same IP subnet: IP address can remain same Switch: which AP is associated with H1? self-learning: learning: switch will see frame from H1 and remember which switch port can be used to reach H1 But what if there are more than one switch in the IP subnet? BBS 1 AP 1 router L2 forwarder (bridge), e.g. an Ethernet Switch AP 2 H1 BBS 2 Same IP subnet

42 Roaming When STA experienced No or bad connection (e.g. poor SNR) perform: rm: Scanning scan the environment, Either passively listen into the medium for beacon signals or Actively send probes into the medium and wait for an AP to answer Reassociation Request station sends a request to one or several AP(s) Reassociation Response success: AP has answered, station can now participate failure: continue scanning AP accepts Reassociation Request signal the new station to the distribution system the distribution system updates its data base (i.e., location information) typically, the distribution system now informs the old AP so it can release resources using yet-to to-be standardized Inter Access Point Protocol (IAPP)

44 Some current IEEE Standardization activities e: MAC Enhancements QoS almost done Enhance the current MAC to expand support for applications s with Quality of Service requirements, and in the capabilities and efficiency of the protocol f: Inter-Access Point Protocol (IAPP) experimental best current practice (not a binding standard) is available Establish an Inter-Access Point Protocol for data exchange via the distribution system i: Enhanced Security Mechanisms completed recently Enhance the current MAC to provide improvements in security n: High Throughput (100Mbps+) ongoing r: Fast (Seamless) Handover support - ongoing u: Interworking between and non Wireless networks s: Mesh Networking ongoing Using a self-configurable wireless infrastructure within an ESS

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